1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to a structure of a regulation member for regulating a position of a leading edge in a sheet feeding direction of a sheet bundle supported by a sheet supporting portion.
2. Description of the Related Art
A conventional image forming apparatus such as a printer, a copying machine, or a facsimile includes a sheet feeding apparatus for separating sheets one by one and feeding each of the sheets to an image forming portion. As the conventional sheet feeding apparatus, for example, as described in U.S. Pat. No. 6,260,840, there is known a sheet feeding apparatus of the following type. Specifically, the sheet feeding apparatus feeds the sheets, which are placed in a tilted tray, one by one through a paper feed roller, and regulates a position of the sheets in such a manner that a leading edge of a sheet bundle is caused to abut against a portion of the tray, the portion being situated on a downstream side in a sheet feeding direction. Note that, in the sheet feeding apparatus, a tilted portion is provided in vicinity of the paper feed roller of an abutment portion against which the leading edge of the sheet bundle abuts. In this manner, the upper sheets enter into a side of the paper feed roller more easily. As the sheet feeding apparatus for feeding the sheets, which are placed in the above-mentioned tilted tray, one by one through the paper feed roller, the following manual feeding apparatus is generally used. Specifically, the manual feeding apparatus is provided on a side surface of an image forming apparatus main body, for feeding the sheets manually placed therein. Note that, for the manual feeding apparatus, stronger demand from users is posed on easily feeding sheets of various types (size, surface property, thickness, material, and the like) rather than feeding a large number of sheets of the same type.
FIG. 4 is a view illustrating a structure of the conventional manual feeding apparatus described above. A sheet S is placed on a tilted tray 42. When a feeding signal is received from a controlling unit (not shown), a pressure plate 42a, which is provided on a side of a proximal end of the tray 42, rotates to a direction indicated by the arrow E. As a result, due to an elastic force of a spring 43, an upper surface on a side of a leading edge of a sheet bundle SA is pressed against a sheet feed roller 51. Next, when the sheet feed roller 51 is caused to rotate, an uppermost sheet S of the sheet bundle SA is fed. Note that, by causing the pressure plate 42a to rotate in a direction opposite to the direction indicated by the arrow E to thereby return to an initial position after one sheet S is fed as described above, the double feed of sheets S is prevented. Here, when the sheet S is fed by the sheet feed roller 51, two or more sheets S may be fed at one time. In this regard, in order to prevent the double feed of sheets S, the conventional sheet feeding apparatus incorporates a torque limiter, and brings a separation roller 53 into pressure contact with the sheet feed roller 51, the separation roller 53 rotating in a sheet returning direction. The above-mentioned separation roller 53 is provided, and hence, in a case where the two or more sheets are fed, it is possible to separate the sheets fed together at one time, one by one due to a returning force of the separation roller 53 and to feed each of the sheets.
In the conventional sheet feeding apparatus described above, when a user places the sheet bundle on the tray, the sheet bundle is slid downward by gravity along the declination of the tray because the tray is tilted. Then, as illustrated in FIG. 5A, the leading edge of the sheet bundle SA abuts against a leading edge regulation member 54 of the tray 42, the leading edge regulation member 54 being provided on the downstream in the sheet feeding direction. With this, a stand-by position of the sheet bundle SA before feeding is determined. Note that, when the feeding signal is thereafter received as described above, the pressure plate 42a rotates in the direction indicated by the arrow E of FIG. 5A, and presses the upper surface on the side of the leading edge of the sheet bundle SA against the sheet feed roller 51. The sheet feed roller 51 is caused to rotate to feed the uppermost sheet.
Here, in a case where a frictional coefficient of the surface of the sheet is low, or in a case where a smoothness of the surface of the sheet is high, the sheets may stick to each other. In this case, even if the sheet feed roller 51 is caused to rotate in a direction indicated by the arrow F of FIG. 5B, it is sometimes impossible to smoothly separate and feed the uppermost sheet S of the sheet bundle SA. In particular, in a case where a smoothness of the surface of the sheet is high, an attractive force between the sheets increases, and hence it becomes difficult to separate the sheet bundle. Further, the above-mentioned tendency becomes more marked under a high-temperature and high-humidity environment.
Further, in a case where the sheet is cardboard having a basis weight of more than 250 g/m2, or in a case where the sheet is a sheet having a size larger than an A3 size, the sheet bundle SA to be stacked on the tray 42 becomes heavier. Further, as the sheet bundle SA becomes heavier, there decreases a pressing force of pressing the sheet against the sheet feed roller 51 by use of the spring for biasing the pressure plate 42a. As a result, a conveying force (force of feeding the sheet) through the sheet feed roller 51 decreases, and feeding failure occurs more easily when the sheets stick to each other. Note that, though a countermeasure thereof is taken by increasing the elastic force of the spring 43, there is a fear that a so large number of sheets are fed together at one time that a separation portion on the downstream is incapable of separating the sheets in a case where thin paper having low basis weight is used. That is because, in the case where the thin paper having the low basis weight is used, the pressing force of pressing the sheet against the sheet feed roller 51 increases and thus the force of feeding the sheet becomes extremely large.
In addition, as illustrated in FIG. 5A, the leading edge of the sheet bundle SA abuts against the leading edge regulation member 54. Therefore, as the sheet bundle SA becomes heavier, also an abutting pressure between the sheet bundle SA and the leading edge regulation member 54 increases. Further, when the abutting pressure increases as described above, a frictional resistance between the leading edge of the sheet bundle SA and the leading edge regulation member increases. Thus, the pressure plate 42a cannot be smoothly moved anymore in the direction indicated by the arrow E. As a result, there may pose a problem that the pressing force decreases or that a point in time of pressing the sheet against the sheet feed roller 51 is delayed. In this case, the sheet feeding failure occurs more easily when the sheets stick to each other. In addition, in the image forming apparatus, there is a demand of decreasing occupation area thereof. Therefore, there is tendency to increase a tilted angle (angle with respect to a horizontal plane) of the tray 42 of the manual feeding apparatus provided on the side surface of the apparatus main body, to thereby decrease a protruding amount of the tray 42. However, when the tilted angle is increased as described above, the abutting pressure of the leading edge of the sheet bundle with respect to the leading edge regulation member 54 increases, and hence the feeding failure occurs more easily.